Elastomeric fluoropolymers (i.e. fluoroelastomers) exhibit excellent resistance to the effects of heat, weather, oil, solvents and chemicals. Such materials are commercially available and are most commonly either copolymers of vinylidene fluoride (VF2) with hexafluoropropylene (HFP) or copolymers of VF2, HFP, and tetrafluoroethylene (TFE).
Other common fluoroelastomers include the copolymers of TFE with one or more hydrocarbon olefins such as ethylene (E) or propylene (P), and also the copolymers of TFE with a perfluoro(alkyl vinyl ether) such as perfluoro(methyl vinyl ether) (PMVE).
Many fluoroelastomers require incorporation of a cure site monomer into their polymer chains in order to crosslink efficiently (Logothetis, A. L., Prog. Polym. Sci., Vol. 14, pp 251-296 (1989); A. Taguet et al. Advances in Polymer Science, Vol. 184, pp 127-211 (2005)). Without such a cure site monomer, the fluoroelastomer may not react at all with curing agents, it may only partially react, or reaction may be too slow for use on a commercial scale. Seals made from poorly crosslinked elastomers often fail sooner than might otherwise be expected. Unfortunately, disadvantages are associated with many of the cure site monomers and curatives in use today. For example, some curatives (e.g. diamines) are toxic. Cure site monomers which contain reactive bromine or iodine atoms can release byproducts during the curing reaction that are harmful to the environment. Other cure site monomers (e.g. those which contain double bonds at both ends of the molecule) may be so reactive that they disrupt polymerization of the fluoroelastomer by altering the polymerization rate, terminating polymerization, or by causing undesirable chain branching, or even gelation to occur. Lastly, incorporation of a cure site monomer into a fluoroelastomer polymer chain may negatively impact the properties of the fluoroelastomer (both physical properties and chemical resistance).
There exists a need in the art for new fluoroelastomer cure systems which are environmentally friendly, do not disrupt polymerization and which do not detract from the properties of the fluoroelastomer.
Telechelic difunctional low molecular weight (number average molecular weight between 1000 and 25,000 daltons) copolymers of vinylidene fluoride (VF2) with perfluoro(methyl vinyl ether) (PMVE) and difunctional copolymers of tetrafluoroethylene (TFE) with PMVE have been disclosed in US 20090105435 A1. A functional group is located at each end of the copolymer main chain. Functional groups disclosed include iodine, allyl, hydroxyl, carboxyl and nitrile.